Jesus condemns divorce and remarriage explicitly in all four Gospels.
"He said to them, 'Because of your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.” - Matthew 19:8-9
"And he said to them, 'Whoever divorces his wife and marries another commits adultery against her, and if she divorces her husband and marries another, she commits adultery.'” - Mark 10:11-12
"Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery." - Luke 16:18
"...for you have had five husbands, and the one you now have is not your husband. What you have said is true.” - John 4:18
Here is St. Paul affirming as well...
" For a married woman is bound by law to her husband while he lives, but if her husband dies she is released from the law of marriage. Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is free from that law, and if she marries another man she is not an adulteress." - Romans 7:2-3
"A wife is bound to her husband as long as he lives. But if her husband dies, she is free to be married to whom she wishes, only in the Lord." - 1 Cor 7:39